Conventional modems, such as V.34 modems, treat the public switched telephone network (PSTN) as a pure analog channel even though the signals are digitized throughout most of the network. In contrast, pulse code modulation (PCM) modems take advantage of the fact that most of the network is digital and that typically central site modems, such as those of internet service providers and on-line services, are connected to the PSTN via digital connections (e.g., T1 in the United States and E1 in Europe). First generation PCM modems transmit data in PCM mode downstream only (i.e., from a central site digital modem to an analog end user modem) and transmit in analog mode, e.g. V.34 mode, upstream (i.e., from the end modem to the central site modem). Second generation PCM modems will also transmit data upstream in PCM mode.
With PCM downstream, the central site PCM modem transmits over a digital network eight bit digital words (octets) corresponding to different central office codec output levels. At the end user's central office, the octets are converted to analog levels which are transmitted over an analog loop. The end user's PCM modem then converts the analog levels into equalized digital levels. The equalized digital levels are ideally mapped back into the originally transmitted octets and the data the octets represent. With PCM upstream, the end user PCM modem transmits analog levels over the analog loop corresponding to the data to be transmitted and the levels are quantized to form octets by a codec in the end user's central office. The codec transmits the octets to the PCM central site modem over the digital network.
However, due to impairments in the digital network, such as digital trunk loss (in the US, typically 0, 3 or 6 dB) caused by digital padding and robbed bit signaling (hereinafter referred to as RBS), caused by the networks in-band signaling, the octets transmitted both in the upstream and downstream directions may be corrupted. If not accounted for, this can cause high data error rates in the modems.
Many modern digital networks which may carry PCM modem data are constructed as T-carrier systems that use robbed bit signaling. The digital data transmitted over these networks is grouped into octets (eight (8) bits) and the octets are grouped into frames (twenty four (24) octets). In FIG. 1 there is shown a frame 10 containing twenty four octets, O.sub.0 -O.sub.23 The frames transmitted over the network are continuous, and the single 24 octet frame 10 is shown for descriptive convenience only. Certain octets are affected by RBS. The network uses the least significant bit (LSB) position of the affected octets to carry data to perform control functions in the network. Thus, for example, the first octet, O.sub.0, may be affected by a type of RBS that forces the LSB of that octet to one, odd RBS, as indicated by the "F" in that octet. (The designation "NC" means "no change"). Depending on the octet of data carried in that interval, RBS may change that octet's data. In particular, if that octet had a zero in its LSB, RBS alters that octet. If, however, that octet had a one in its LSB, RBS would have no affect on the octet from the end-use's perspective.
It has been observed that RBS has deterministic periodicity with periods of six or twenty four octets. In this example, the RBS period is twenty four. Since RBS recurs every twenty four octets, the octets can be viewed as appearing in a basic period 12 of twenty four time slots or intervals, 0-23, which may or may not be affected by RBS. For example, octet O.sub.0 appears in slot "0" which is affected by RBS, while octet O.sub.1, for example, appears in slot "1" and is unaffected by RBS. It should be noted that due to the nature of the networks, it is possible to have more than one RBS affected interval in the basic period of twenty four, as evidenced by affected octet O.sub.6.
Methods for detecting and mitigating downstream digital impairments are known. Examples of these methods are described in the following co-pending applications, assigned to the assignee of the present invention: U.S. application Ser. No. 08/885,710, entitled System, Device and Method for Detecting and Characterizing Impairments in a Communication Network; filed Jun. 30, 1997; U.S. aplication Ser. No. 08/730,433, entitled System And Device For, And Method Of, Detecting, Characterizing. And Mitigating Deterministic Distortion In A Communications Network, filed Oct. 15, 1996; U.S. application Ser. No. 08/979,994, entitled, System. Device and Method for Detecting Impairments in a Communication Network, filed Oct. 26, 1997; and U.S. application Ser. No. 08/979,196, entitled, Apparatus, System And Method For Transmitting And Receiving A Training Sequence Optimized For Detecting Impairments In A Communication Network, filed Oct 26, 1997. These applications are incorporated herein in their entireties.
With downstream transmission, the points transmitted over the digital network are known and this information is used for digital impairment detection according to the above referenced applications. However, with upstream transmission, before determining digital impairments, downstream PCM echo and the characteristics of the analog loop (channel), the points transmitted over the digital channel are unknown and the techniques for detecting digital impairments described in the above applications are not applicable to upstream transmission.
Therefore, a need exists for a device and method for detecting PCM upstream digital impairments to improve PCM upstream transmission. This information can also be used to estimate analog channel characteristics and PCM downstream echo to further improve PCM upstream transmission.